The present invention relates generally to tire retreading; and more particularly, it relates to novel and improved curing envelope sealing rings for use with an outer curing envelope to enclose a tire casing and tread strip assembly during retreading.
Tire retreading, or recapping, may be applied to any size tire casing, but is more commonly used on large radial truck tires. The casings of these tires usually have a longer useful life than their treads. Therefore, it is often more cost effective to retread a worn casing than to replace it with a new tire.
One known process simultaneously bonds and vulcanizes an uncured rubber on a worn tire casing in a tread forming mold. In another known process, a precured rubber strip with a premolded tread is wrapped around the buffed crown surface of the worn tire casing with a bonding agent such as uncured rubber or cushion gum disposed therebetween. The assembly is then sealingly enclosed in an annular elastomeric curing envelope of U-shaped radial cross-section, typically of thin synthetic rubber, or the like, such as manufactured by Presti Rubber Products, Inc. of New Britain, Pa. The annular space between the tread and the envelope is evacuated, thereby stretching the outer envelope into intimate contact with the outer sidewalls of the casing and all surfaces of the tread strip. This insures that uniform pressure is applied over the entire bonding area without tread distortion during the curing process. The outer envelope also prevents either air or steam from being entrapped between the tread strip and the casing and migrating into the bonding agent. The entire assembly is then placed in a curing chamber, or autoclave, at elevated temperatures and pressures to vulcanize the uncured components and to positively bond the casing and tread strip together. The outer envelope is then removed for reuse if desired.
Outer curing envelopes as described are not as easy to install as desired, especially when smaller inside diameter envelopes need to be pulled over the outside diameter of large truck tire casings. Therefore various special mountings, rims, and linings have been developed to facilitate sealing of the curing envelope around the tire casing and tread assembly.
In one known commercially available arrangement, the side skirts of the outer curing envelope are made longer than the sidewalls of the tire casing with which it is to be used. The excess margin of the outer envelope, i.e. the margins adjacent to the inner peripheries of the skirts, are tucked inside the tire beads and held in place by a radially-expandable ring. In this arrangement, the inside surface of the tire casing is exposed while the entire outside is encased.
Another arrangement involves the use with an outer envelope of an inner envelope of non-form fitting, pieced together construction for enclosing the entire inside surface and beads of the casing. This inner envelope includes annular flaps which are overlapped by the skirts of the outer curing envelope on both sides of the tire casing. The entire tire casing is thereby sheathed.
An improvement in the inner envelope arrangement described above is disclosed in co-pending U.S. patent application Ser. No. 07/336,758 filed on Apr. 12, 1989, by Frederick J. Presti, now U.S. Pat. No. 5,007,978, owned by the assignee of the present application. In this improvement, the inner envelope is of tripartite construction having a U-shaped portion which engages the inside of the tire casing and a pair of molded form-fit assemblies carried on the skirts of the U-shaped portion. The molded form-fit assemblies are each constructed so that the base and flaps conform to the surface contour of the beads of the casing. The flaps extend radially outward and along the outer sidewalls of the tire casing and terminate in enlarged annular ribs which sealingly underlap the skirts of the outer envelope on both sides of the casing. While very useful for the purposes intended, this improved inner envelope must be provided in numerous sizes and shapes to satisfy the large variety of sizes and shapes of tire casings. For reasons to be stated, there is a need for a retreading system which obtains the advantages associated with the use of an outer envelope but for a wider range of tire sizes.